Cosmic dust grains found on city rooftops for the first time

Cosmic dust raining down from space has been discovered on rooftops in three major cities.

The tiny particles date back to the birth of the solar system 4.6 billion years ago.

Scientists usually collect cosmic dust in the frozen wastes of Antarctica. Now, for the first time, the space debris has been found hidden in city dirt.

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Researchers sifted through 300 kilograms of muck trapped in roof gutters in Paris, Oslo and Berlin. Using magnets to pull out the particles, which contain magnetic minerals, they identified a total of 500 cosmic dust grains.

“We’ve known since the 1940s that cosmic dust falls continuously through our atmosphere, but until now we’ve thought that it could not be detected among the millions of terrestrial dust particles, except in the most dust-free environments such as the Antarctic or deep oceans,” says Matthew Genge at Imperial College London. “The obvious advantage to this new approach is that it is much easier to source cosmic dust particles if they are in our backyards.”

The idea of looking for cosmic dust in the city was the brain child of amateur scientist Jon Larsen, from Norway, who contacted Imperial.

“When Jon first came to me I was dubious,” Genge says. “Many people had reported finding cosmic dust in urban areas before, but when they were analysed scientists found that these particles were all industrial in origin.”

City cosmic dust was found to be larger than previously recovered particles, measuring around 0.3 millimetres across instead of the more usual 0.01 millimetres. In addition the dust found in cities contained fewer feather-like crystals than the much more ancient particles from Antarctica.

The differences may be linked to changes in the orbits of planets such as the Earth and Mars over millions of years, Genge says. Resulting gravitational disturbances may have influenced the trajectory of the particles as they hurtled through space. This in turn would have an effect on the speed at which they slam into the Earth’s atmosphere and heat up.

“This find is important because if we are to look at fossil cosmic dust collected from ancient rocks to reconstruct a geological history of our solar system, then we need to understand how this dust is changed by the continuous pull of the planets,” Genge says.

The study showed that the city cosmic dust must have entered the atmosphere at around 12 kilometres per second. This makes them the fastest moving dust particles ever found on Earth.